Inductive heating process control of continuous cast metallic sheets

ABSTRACT

An apparatus for rolling of a powder metallurgical metallic workpiece is provided having a feeding device, a first induction heating apparatus in operable communication with a first RF generator and at least a first hot rolling mill. A process control device monitors at least one parameter of the first RF generator and outputs a signal. A method for continuous rolling of a metallic workpiece is also provided using the apparatus for rolling. The process control device signal can be used to monitor metallurgical properties of the workpiece and provide in-line evaluation of the workpiece.

BACKGROUND

1. Field of the Invention

The invention is directed to the manufacture of metallic products suchas sheet, strip, rod, wire or band, especially of difficult to workintermetallic alloys like aluminides of iron, nickel and titanium.Particularly, the invention is directed to a hot rolling operationutilizing an inductive heating processing step and associated processcontrol feedback.

2. Background of the Invention

In the description of the background of the present invention thatfollows, reference is made to certain structures and methods, however,such references should not necessarily be construed as an admission thatthese structures and methods qualify as prior art under the applicablestatutory provisions. Applicants reserve the right to demonstrate thatany of the referenced subject matter does not constitute prior art withregard to the present invention.

Powder metallurgical processes for preparing sheets from a powder havingan intermetallic alloy composition such as an iron, nickel or titaniumaluminide, are disclosed in commonly owned U.S. Pat. No. 6,030,472, thecontents of which are herein incorporated by reference. In suchprocesses, non-densified metal sheets of aluminide are consolidated froma powder by roll compaction, tape casting or plasma spraying. A coldrolled sheet is formed by cold rolling the non-densified metal sheet soas to increase the density and reduce the thickness followed byannealing.

U.S. Pat. No. 6,143,241, the contents of which are herein incorporatedby reference, discloses a method of manufacturing intermetallic productsby a process including cold working and intermediate or final flashannealing.

U.S. Pat. No. 6,166,360, the contents of which are herein incorporatedby reference, discloses the use of a combined induction heating andconduction heating process to uniformly heat a part of varying thicknessfor hot forming and heat treatments such as hardening and tempering.

U.S. Pat. No. 4,782,994, the contents of which are herein incorporatedby reference, discloses an apparatus for inline annealing of stripmaterial including a hot press followed by a plate annealer and a fieldannealing process. A computerized control means provides signals for astrip feeding rate, temperature and pressure of the heated pressurerolls, temperature of the separate inline annealer, and tension on thestrip.

Iron aluminides generally require multiple pass rolling operationsduring which a sheet stock is reduced in thickness. The capitalinvestment required to assemble a rolling line for iron aluminide issubstantial. Currently, aluminide stock is processed in a multiple rollrolling line having up to sixty or more rolling operations. Forinstance, product that has undergone multiple rolling steps is removedfrom the in-line process and tested off-line. Testing techniques may benon-destructive (i.e., ultrasonic examination) or may be destructiveevaluation processes (i.e., sectioning and polishing for microscopic orvisual examination). Thus, metallurgical evaluation is made as to theacceptability of the final work product only after completion of thevalue added processing steps.

Therefore, there is a need to provide a rolling operation for aluminidematerials, particularly sheet stock, that reduces the capital investmentand provides for inline process monitoring and feedback. Further, thereis a need to minimize the number of required passes in a rollingoperation for aluminide stock material.

SUMMARY OF THE INVENTION

It is an object of the present invention to minimize the number ofrolling steps required to reduce a workpiece by a minimum of 50% of itsthickness.

It is a further object of the present invention to provide a hot rollingline for the reduction of aluminide metallic stock.

An additional object of the present invention is to provide inlinemonitoring and feedback capability of final product metallurgicalproperties.

An apparatus for rolling of a metallic workpiece is provided having afeeding device operable to convey the workpiece along the process line,a first induction heating apparatus in operable communication with afirst RF generator and operable to heat the workpiece during movementalong the process line, at least a first hot rolling mill and a processcontrol device operable to monitor at least one parameter of the firstFR generator. An optional second hot rolling mill with associatedinduction heating apparatus and RF generator can be included. A heatingdevice in thermal communication with one or more surfaces of a roll ofthe first and/or second hot rolling mill can maintain the surface of theroll at a second temperature. Also, optionally, at least one coldrolling mill can be located downstream of a last hot rolling millfollowed by a post-roll annealing apparatus having an annealinginduction heating apparatus in operable communication with an annealingRF generator.

A method for continuous rolling of a metallic workpiece is providedusing the apparatus for rolling and having the steps of feeding theworkpiece at a first speed into a process line, heating the workpieceduring movement along the process line to a first temperature in atleast a first inductive heating apparatus in operable communication witha first RF generator, monitoring at least one parameter of the first RFgenerator, and hot rolling the workpiece in at least a first hot rollingmill operating at a second speed. Multiple hot rolling operations canoptionally be used followed by cold rolling and/or annealing. Theresulting workpiece can have a density greater than 95%.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Other objects and advantages of the invention will become apparent fromthe following detailed description of preferred embodiments inconnection with the accompanying drawings in which like numeralsdesignate like elements and in which:

FIG. 1 is a schematic illustration of a first embodiment of an aluminidehot rolling line layout; and

FIG. 2 is a schematic illustration of a second embodiment of analuminide hot rolling line layout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic illustration of an embodiment of the layout of ahot rolling line 100 for the processing of an aluminide workpiece 102. Aworkpiece 102, such as sheet stock, can be fed into a feeding device 104which outputs to a first induction heating apparatus 106. The heatedworkpiece 102 is then fed into a first hot rolling mill 108 forreduction in thickness. A first RF generator 110 in operablecommunication with the first induction heating apparatus 106substantially constitutes a first induction heating zone 112.

The feeding device 104 can be any suitable device that can maintain thedesired tension and lateral alignment on the workpiece 102.Alternatively, the feeding device 104 can be eliminated and a suitablefeeding means can be employed to achieve the desired tension and lateralalignment, for example by a continuous feeding operation conducted froma tensioned bobbin. In a preferred embodiment, the feeding device 104can be a pinch roll that operates at a first speed that is slower than asecond speed at which the hot rolling mill 108 operates, therebymaintaining the workpiece 102 in tension through the induction heatingzone 112. A suitable first speed is approximately 30 ft/sec.Additionally, the pinch roll can contribute to the lateral positioningof the workpiece 102 as it is fed into the induction heating zone 112.

Typical temperatures maintained inside the induction heating zone 112are sufficient to raise the temperature of a workpiece 102 above arecrystallization temperature. For example, for iron aluminide,temperatures are maintained between 700-1100° C. This value represents a10-20% over capacity for the anticipated power requirement for a 12″wide, 26 mils thick iron aluminide workpiece at 80% density. Apreferable value for workpiece temperature is from 700-1000° C.

Aluminides, including iron aluminides, can react with oxygen to formoxides. This is more prevalent at elevated temperatures that canincrease the reactivity of the aluminide and make the reaction morefacile. Thus, for example, cold rolling methods can help to mitigate thereactivity of aluminides compared to hot rolling methods. Additionalmitigating techniques include the use of inert atmospheres or vacuumprocesses during operations at elevated temperatures. In the exemplaryhot rolling line depicted in FIG. 1, an inert atmosphere can bemaintained in the induction heating zone 112. Optionally, the same orsimilar techniques to reduce the oxygen content can be used not only inthe induction heating zone 112, but also during any operation conductedat elevated temperatures, such as hot rolling and/or annealing.

In the hot rolling mill 108, the workpiece is reduced approximately 50%in a single pass. The hot rolling mill 108 can be a 2 high or 4 highrolling mill, as is known in the art. Optionally, the hot rolling mill108 can be preheated to a temperature high enough to preheat the rolland low enough not to damage the roller's packing material and thedrive. A suitable temperature is between 300° C. and 700° C. Highpreheating temperature can be tolerated if required, by usingnon-conventional rollers and/or using coolant to cool the drives. Thepreheating of one or more roll surfaces results in a reduced thermaldriving force for cooling during the rolling operation.

Any suitable means of heating the roller surface can be used. Forexample, an IR heating lamp can be placed in thermal communication withthe roller surfaces at a position not to interfere with the rollingoperation. One particular position can be at an opposite end of adiameter of the rolling surface from where the rolling surface meets theworkpiece. Additional heating means include inductive heating and/or ahybrid of inductive and IR heating.

The workpiece 102 can be any suitable metallic workpiece. For example, ametallic workpiece can be a binary compound or alloy of an aluminide ofiron, titanium, or nickel. Preferably, the metallic workpiece is analuminide of at least 75% density, and preferably greater than 80%density. Examples of suitable iron aluminides can be found in U.S. Pat.No. 6,030,472, the contents of which are herein incorporated byreference.

The workpiece may be in the form of a continuous strip or a batch strip.For example, sheet stock formed in a compaction operation with a bindermay be formed into a semi-dense sheet and, after the binder is removedby, for example, heating in an oven up to approximately 600° C., can becold compacted or cold rolled to a sheet having a desired maximumthickness and minimum density. In one example, the metallic workpiece isan iron aluminide sheet stock that is 12″ wide having a thickness of 26mils and a density of 80%. This metallic workpiece can be cut intostrips for a batch process or may be fed continuously into the pinchroller.

FIG. 2 is a schematic illustration of an embodiment of the layout of ahot rolling line 200 utilizing at least one hot rolling operation withthe addition of a cold rolling operation and a post-rolling annealingoperation. A workpiece 202, such as sheet stock, can be fed into a firstinduction heating apparatus 206. The heated workpiece 202 is then fedinto a first hot rolling mill 208 for reduction in thickness. A first RFgenerator 210 in operable communication with the first induction heatingapparatus 206 substantially constitutes a first induction heating zone212.

Optional, multiple hot rolling operations can be used in the hot rollingline 200. In the exemplary embodiment of FIG. 2, the workpiece that hasmade one pass through the first hot rolling mill 208 can be fed into asecond induction heating apparatus 216. The heated workpiece 202 is thenfed into a second hot rolling mill 218 for reduction in thickness. Asecond RF generator 220 in operable communication with the secondinduction heating apparatus 216 substantially constitutes a secondinduction heating zone 222. It should be understood that any number ofhot rolling mills with associated induction heating zones can beutilized to obtain a desired reduction in thickness of a workpiece.

A cooling zone 224 positioned after the last hot rolling mill allows theworkpiece 202 conveyed by the hot rolling line 200 to cool to a suitabletemperature for subsequent handling, such as being taken up by areceiving bobbin 252, or optional cold working operations and/orannealing. In the embodiment shown in FIG. 2, the hot rolling operationsare performed sequentially. After all hot rolling operations arecompleted, an optional cold rolling operation can be performed followedby a post-roll annealing operation. A cold rolling mill 226 can coldwork the workpiece 202 to further reduce the thickness, provide adesired surface finish, provide a particular stress, or effect acrystallographic change, such as grain size refinement.

Annealing can be performed in an annealing induction apparatus 228 inoperable communication with an annealing RF generator 230 and thatsubstantially constitutes an induction annealing zone 232. The annealingoperation is optional and can be used to obtain a desired property inthe workpiece, such as a surface finish or reduction in internalstresses. If annealing is performed as a finishing operation, a coolingzone 240 allows the workpiece 202 conveyed by the hot being taken up bya receiving bobbin 252.

Alternatively, any number of hot rolling operations can be interspersedwith any number of cold rolling operations and/or post roll annealingoperations to provide the desired workpiece properties includingthickness, surface finish, crystal structure, and internal stresses.

The workpiece 202 of the hot rolling line can be a feed stock storedand/or transported on a supply bobbin 250 and which is unwound and fedinto the first induction heating zone 212. The supply bobbin 250, incombination with the feed rate and the first hot rolling mill 208, canprovide the feeding means to maintain the workpiece 202 in tension atleast through the first induction heating zone 212 and to providelateral alignment of the workpiece.

Alternative continuous feeding operations can be used to providematerial for the hot rolling line. For example, it should be understoodthat a continuous feeding operation can also include the use of feedingand compacting equipment directly forming a workpiece that is fed intothe hot rolling line. Alternatively, batch operations can be used tosupply material to the hot rolling line.

Each subsequent induction heating zone can be co-located with a hotrolling mill to provide additional reductions. For example, a 26 mil 80%dense iron aluminide feedstock can be reduced 50% to approximately 13mils in a first combination hot rolling mill/induction heating unitsimilar to that described for the FIG. 1 embodiment of a hot rollingline layout 100. Further reductions to 7 mils while obtaining a densityof greater than 95%, and preferably greater than 97%, can occur in asecond combination hot rolling mill/induction heating unit similar tothat described for the FIG. 2 embodiment of a hot rolling line layout200. Further, additional inductive heating zones and hot rolling millsmay be implemented in a hot rolling line to obtain any desirable finalthickness of feedstock.

A cold rolling mill can be utilized as shown in FIG. 2. A cold rollingmill can provide a surface finish to the workpiece. This surface finishcan increase the strength and yield of the workpiece. Generally, coldrolling is followed by a post-roll annealing operation. This step isprovided to remove internal stresses and increase the strength and theyield of the workpiece. In FIG. 2, the post roll annealing step isprovided by an induction heating apparatus operatively coupled to an RFgenerator. The principles and purposes of the inductive heatingapparatus are similar to those previously discussed. An example of coldworking and annealing and the impact upon aluminide sheets is presentedand discussed in commonly owned U.S. Pat. No. 6,143,241, the contents ofwhich are herein incorporated by reference.

The induction heating apparatus can be built to a size and shape toaccommodate the shape of the workpiece or a portion of the workpiece tobe heated. The induction heating apparatus has an inductor which can bea simple coil or several heaters or coils connected and automated toprovide a continuous supply of heated stock exiting the inductionheating zone. Electric current in the inductor is carried by watercooled copper tubing from an RF generator. The copper inductor isinsulated from the workpiece by refractory material or by fittedbrickwork. Preferably, the RF generator and the inductor are placed neareach other to minimize current losses.

Two modes are available in induction heating. A first mode is calledlongitudinal heating (or solenoid heating). In longitudinal heating, acoiled inductor tube is formed as a helix into which a workpiece may bepassed such that the coiled tube spirals around the workpiece. Thisgeometry can produce a very uniform temperature distribution and canheat the workpiece from the center.

A second mode of induction heating is called transverse flux heating(TFX). In transverse flux heating, a planar coiled tube is used to formthe magnetic field. The magnetic field is planar with a Gaussiandistribution; therefore, it is not as uniform as the field form inlongitudinal heating. This can adversely impact the distribution at theedges of a workpiece and can lead to non-uniform heating. However,appropriately positioned susceptors can modify the magnetic field toform a more Gaussian distribution centered on the center line of theworkpiece.

The selection of a particular inductive heating mode can be made bybalancing power requirements with cost considerations. Longitudinalheating can be more expensive and generally operates at higher powerstranslating to higher operating costs. Additionally, workpiece geometrycan influence the selection of an inductive heating mode. For example, asheet geometry can be more appropriately heated in a TFX mode and a tubeor rod or wire can be more appropriately heated in a longitudinalheating mode.

The RF generator associated with the induction heating apparatusprovides power at a particular frequency to effect the inductive heatingprocess. Both power and frequency can vary in proportion to certainproperties of a metal workpiece within the inductive heating apparatusas inferred from the permeability of the workpiece. Methods in whichinductive heating parameters are monitored are disclosed in U.S. Pat.Nos. 4,816,633, 4,897,518, and 5,630,957, the disclosures of which areherein incorporated by reference.

Hans G. Matthes, Novel Process of Quality Control During InductiveHardening Process, presentation for CIT/FNA at the Induction HeatingClinic conducted at the Furnace 2000 Trade Show, Orlando, Fla., Mar.29-30, 2000, the contents of which are herein incorporated by reference,presents a discussion on the use of an integrated process monitoringdevice with a RF generator of an inductive heating apparatus. In theinduction heating process, the induced effective power (P_(w)) and thefrequency (f) can be monitored in an on-line process and the valuescompared to stored predetermined values associated with and developedfor a particular heating regime of a workpiece. A process controller canmake a binary decision as to whether or not the workpiece is within theestablished metallurgical standards determined to be acceptable by theoperator and stored by the controller.

Comparison of monitored and stored values can be effective indetermining metallurgical characteristic properties of the workpiecebeing heated as well as can be related to physical defects andabnormalities in the workpiece. For example, the relationship betweenP_(W) and effective power P_(eff) can be related to delamination arisingin the induction heating apparatus, change in diameter and/or size of aworkpiece, eccentricity, surface cracking, and other materialcharacteristics that can be inferred from changes in permeability of aworkpiece.

Process monitoring can be used in conjunction with any inductive heatingoperation of a hot rolling line. For example, in FIG. 1, the first RFgenerator 110 in operable communication with the first induction heatingzone 112 can have inductive heating parameters, such as P_(eff) and f,monitored by a process monitor 114. The results from the monitoring canbe used to evaluate the properties of the workpiece on-line. A workpieceor section of a workpiece that is determined to not have the desiredproperties can then be marked for non-use, such as recycling, scrap, orfurther evaluation, without disrupting the hot rolling operations.Alternatively, other suitable steps can be initiated in response to aworkpiece not having the desired properties, such as feedback to theforming process or initiation of a reject operation.

In FIG. 2, the process monitoring capability can be used in conjunctionwith any inductive heating operation of the hot rolling line 200including monitoring inductive heating parameters, such as P_(eff) andf, by a process monitor 214 that can be associated with the RF generatorof, for example, the inductive heating zones 212, 220. Each inductionheating zone in the process can have an associated process monitor.Alternatively, at least one and optionally, any desired number ofinduction heating zones has an associated process monitoring capabilitywith a process monitor. Further, the process monitoring of the RFgenerator in the post-roll annealing environment can be further used toevaluate workpiece quality and provide feedback to the operator and tothe process.

An additional monitoring technique for the inductive heating zoneutilizes surface temperature of the workpiece. Since the power inputinto the inductive heating zone is calibrated to raise the temperatureto a programed value in a predetermined geometrically shaped workpiece,a variation in workpiece geometry in excess of acceptable manufacturingtolerances can result in deviation from the programmed temperature. Forexample, if a sheet workpiece is not square, that is, one edge isthicker than a second edge, then the thicker edge can be cooler than thethinner edge for a given thermal input. This comparison assumes that theinductive heating apparatus outputs a constant thermal energy.Similarly, a material that is bowed may have areas of its surface thatare hotter or cooler than the programmed temperature. Therefore, asensor equipped to determine thermal profiling can be used to monitortemperature variations on the surface of the workpiece. An example ofsuch a monitor is an IR thermal camera. A suitable temperaturedifferential from edge to edge of a workpiece is approximately 25-50° C.at 1000° C. The IR temperature sensing can be used in any combination ofinductive heating zones or the hot rolling line 100, 200.

The temperature provided to the workpiece in the induction heating zoneprovides for easier working of the workpiece during the hot rollprocess. Additionally, preheating of the hot roll surfaces reinforcesthe preheating step by minimizing heat loss to the rollers. An adequatetemperature to the workpiece can promote plastic deformation in theworkpiece during subsequent rolling operations rather thanembrittlement, cracking, or other non-desired deformation processes. Forexample, in a dislocation controlled deformation process, increasedtemperature results in increased mobility of defects. Increased mobilityof defects results in a reduction in the yield stress required to deformthe workpiece. Similarly, if the temperature in the inductive heatingzone is adequate to cause flowing of the material, then a concomitantdecrease in the yield stress may result.

Although the present invention has been described in connection withexemplary embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without departing from the spiritand scope of the invention as defined in the appended claims.

1-16. (canceled)
 17. A method for continuous rolling of a powdermetallurgical metallic workpiece comprising the steps of: feeding theworkpiece at a first speed into a process line; heating the workpieceduring movement along the process line to a first temperature in atleast a first inductive heating apparatus in operable communication witha first RF generator; monitoring at least one parameter of the first RFgenerator; and hot rolling the workpiece in at least a first hot rollingmill operating at a second speed.
 18. The method of claim 17, whereinthe first temperature is approximately 700-1100° C.
 19. The method ofclaim 18, wherein the first temperature is 700-1000° C.
 20. The methodof claim 17, wherein the at least one parameter is power or frequency.21. The method of claim 17, wherein the first hot rolling mill reducesthe workpiece by at least 50% during movement along the process line.22. The method of claim 17, wherein the first speed is less than thesecond speed such that the workpiece is in tension during the heatingstep.
 23. The method of claim 17, wherein the first speed is 30 ft/sec.24. The method of claim 17, wherein the first inductive heatingapparatus operates in a transverse flux heating mode.
 25. The method ofclaim 17, further comprising the step of: monitoring a surfacetemperature of the workpiece during or directly after heating in thefirst inductive heating apparatus.
 26. The method of claim 17, furthercomprising the step of: preheating at least one roll surface of thefirst hot rolling mill to a second temperature.
 27. The method of claim26, wherein the second temperature is 300-700° C.
 28. The method ofclaim 17, further comprising the steps of: heating the workpiece to thefirst temperature in a second inductive heating apparatus locateddownstream of the first hot rolling mill; monitoring at least oneparameter of a second RF generator operably associated with the secondinductive heating apparatus; hot rolling the workpiece in a second hotrolling mill operating at the second speed.
 29. The method of claim 28,wherein the second hot rolling mill reduces the workpiece to provide adensity greater than 95%.
 30. The method of claim 29, wherein thedensity is greater than 97%.
 31. The method of claim 28, wherein the atleast one parameter is power or frequency.
 32. The method of claim 28,wherein the second hot rolling mill reduces the workpiece by at least50% during movement along the process line.
 33. The method of claim 28,further comprising the step of: monitoring a surface temperature of theworkpiece during or directly after heating in the first inductiveheating apparatus.
 34. The method of claim 17, further comprising thesteps of: cold rolling the workpiece after final hot rolling; andannealing the workpiece at a temperature sufficient to reduce internalstresses and to increase mechanical strength and yield strength.
 35. Themethod of claim 34, wherein the annealing is conducted in an inductiveheating apparatus operatively associated with an annealing RF generator,and wherein at least one parameter of the annealing RF generator ismonitored by a process control device.
 36. The method of claim 35,wherein the at least one parameter is power or frequency.
 37. The methodof claim 17, further comprising the step of: providing an inertatmosphere around the workpiece during the heating in the firstinduction heating apparatus and during the rolling in the first hotrolling mill.
 38. The method of claim 28, further comprising the stepof: providing an inert atmosphere around the workpiece during theheating in the second induction heating apparatus and during the rollingin the second hot rolling mill.
 39. The method of claim 28, furthercomprising the step of: preheating at least one roll surface of thesecond hot rolling mill to a second temperature.
 40. The method of claim39, wherein the preheating is carried out using IR lamps, inductiveheating, or hybrid heating.
 41. The method of claim 28, wherein thesecond inductive heating apparatus operates in a transverse flux heatingmode.
 42. A method for rolling of a powder metallurgical green bodymetallic workpiece, comprising: forming a powder metallurgical greenbody metallic workpiece by a green body compaction operation with abinder and removing the binder by heating; feeding the green bodymetallic workpiece into a process line; heating the green body metallicworkpiece during movement along the process line in an inductive heatingapparatus in operable communication with an RF generator; monitoring atleast one parameter of the RF generator; and hot rolling the workpiecein a hot rolling mill, wherein results of the monitoring are used toevaluate properties of the workpiece.
 43. The method of claim 42,wherein the green body metallic workpiece is cold compacted or coldrolled after the binder is removed by heating.
 44. The method of claim42, wherein the green body metallic workpiece is an aluminide.
 45. Themethod of claim 42, wherein the green body metallic workpiece has adensity of at least 75%.
 46. The method of 17, wherein the workpiece isa strip or sheet.
 47. The method of claim 42, wherein the workpiece is astrip or sheet.
 48. The method of claim 17, wherein the monitoringcomprises comparing a monitored value of the parameter of the first RFgenerator to a stored value of the parameter so as to evaluateproperties of the workpiece.
 49. The method of claim 48, furthercomprising comparing the evaluated properties of the workpiece todesired properties of the workpiece, and providing feedback to a greenbody forming process when the evaluated properties differ from thedesired properties.
 50. The method of claim 42, wherein the monitoringcomprises comparing a monitored value of the parameter of the RFgenerator to a stored value of the parameter so as to evaluate theproperties of the workpiece.
 51. The method of claim 50, furthercomprising comparing the evaluated properties of the workpiece todesired properties of the workpiece, and providing feedback to a greenbody forming process when the evaluated properties differ from thedesired properties.